In recent years, with respect to a demand for reduction in weight of various steel sheets having the purpose of improvement in fuel efficiency of an automobile, thinning through high strengthening of a steel sheet of an iron alloy or the like, application of light metal such as Al alloy or others have been promoted. However, the light metal such as Al alloy has an advantage that specific strength is higher than that of heavy metal such as steel but has a disadvantage that it is remarkably expensive, so that the application thereof is limited to special use. Accordingly, the high strengthening of the steel sheet is required to promote the reduction in weight of various members more inexpensively and extensively.
The high strengthening of the steel sheet is accompanied by a deterioration in a material property such as formability (workability) in general. Therefore, it is important to achieve the high strengthening without degrading material property in development of a high-strength steel sheet. In particular, stretch-flanging workability, burring workability, ductility, fatigue endurance, corrosion resistance, and the like are required of a steel sheet used for automobile members such as an inner sheet member, a structural member, and an underbody member, and it is important how these material properties and strength are exhibited at a high level in a well-balanced manner. For example, tough hole expandability (λ value) is required of a steel sheet used for automobile members such as a structural member and an underbody member, which occupy about 20% of body weight. This is because press forming mainly typified by stretch-flanging and burring is performed after blanking, opening and the like by shearing, punching and the like.
In the steel sheet used for such members, it is concerned that a flaw, a micro-crack, and others occur in an edge formed by the shearing or the punching, and a crack grows due to the generated flaw or micro-crack to cause fatigue fracture. Therefore, in the edge of the steel sheet, it is needed not to cause the flaw, the micro-crack, and the like in order to improve the fatigue endurance. As the flaw, micro-crack, and the like which occur in the edge, a crack is exemplified which occurs in parallel with the sheet surface. The crack is sometimes referred to as peeling. Conventionally, the peeling occurs with a probability of about 80% in a 540 MPa class steel sheet in particular, and occurs with a probability of about 100% in a 780 MPa class steel sheet. Further, the peeling occurs without correlating with a hole expansion ratio. For example, the peeling occurs when the hole expansion ratio is even 50% or even 100%.
For example, as a steel sheet excellent in the hole expandability (λ value), a steel sheet in which the main phase is ferrite and and which is precipitation-strengthened by fine precipitates of Ti, Nb, or the like and a manufacturing method thereof are reported.
A hot-rolled steel sheet having a purpose of high strength and improvement in stretch-flangeability is disclosed in Patent Literature 1. Hot-rolled steel sheets having a purpose of improvement in a stretch and stretch-flangeability are disclosed in Patent Literatures 2 and 3.
However, even by using the hot-rolled steel sheets disclosed in the patent literatures 1 to 3, it is difficult to sufficiently suppress the flaw and the micro-crack on the edge formed by the shearing, punching or the like. For example, the peeling occurs after punching in the hot-rolled steel sheets disclosed in Patent Literatures 2 and 3. A winding condition for manufacturing the hot-rolled steel sheet disclosed in the patent literature 1 is very tough. Moreover, because the hot-rolled steel sheets disclosed in Patent Literatures 2 and 3 contain Mo of 0.07% or more, which is an expensive alloying element, a manufacturing cost is high.